Tips and tricks for posing / overlaying correctly a 3D character over an image mapped on a 2D plane
Instead of using an image on a plane use the image as background and composite later using the compositor.
Put the background image in Front and set the opacity to a value where you can see the image and your object.

Consider a few more things: The settings for the camera in blender will make a difference. If you are trying to match the placement of 3D elements on an image, the point where things match reality will only be valid for a single point in space. If the camera is a bit higher or lower or has a different perspective, then everything you do will be off. In other words the camera settings (field of view, height and rotation) should also match those of the original camera.
Use Blam or similar tools to determine the settings and placement of the camera in blender.
Create very basic proxy auxiliary objects that might guide you (like the couch) and help you visualize things when you are not looking through the camera view.

NOTE: Blam is no longer mantained and has been replaced by Fspy: https://fspy.io/, but there is no importer for blender 2.79 or previous versions.
For a detailed post on how to use blam and Fspy read: How can I recreate geometry using a photograph?